1. Field of the Invention
The present invention relates, generally, to a method of fabricating a printed circuit board (PCB) having embedded multi-layer passive devices, and more particularly, to a method of fabricating a PCB having an embedded multi-layer capacitor, in which a capacitor is formed to have multiple layers in the PCB to increase capacitance.
2. Description of the Related Art
Although discrete chip resistors or discrete chip capacitors have been generally mounted on almost all PCBs to date, PCBs in which passive devices, such as resistors or capacitors are embedded have recently been developed.
In techniques of fabricating PCBs including embedded passive devices, the passive devices, such as resistors or capacitors, are inserted into outer layers or inner layers of the substrate using novel materials and processes, to substitute for conventional chip resistors and chip capacitors.
That is, the PCB including embedded passive devices means that the passive devices, for example, capacitors, are embedded in the inner layers of the substrate or outer layers thereof. Regardless of the size of the substrate itself, if a capacitor, serving as a passive device, is incorporated into the PCB, this is called an ‘embedded capacitor’. Such a substrate is referred to as an ‘embedded capacitor PCB’.
The most important characteristic of the embedded passive device PCB is that the passive device is integrally provided in the PCB without the need for mounting it on the substrate.
In general, fabrication techniques of the embedded passive device PCB are largely classified into three types.
First is a method of manufacturing a polymer thick film type capacitor, including applying a polymer capacitor paste, which is then heat cured, that is, dried, to realize a capacitor.
Specifically, this method includes applying the polymer capacitor paste on the inner layer of a PCB, and drying the applied polymer capacitor paste, on which a copper paste is then printed and dried to form an electrode, thereby obtaining an embedded capacitor.
Second is a method of manufacturing an embedded discrete type capacitor, including coating a PCB with a ceramic-filled photodielectric resin, which has been patented by Motorola Co. Ltd., USA.
The above method includes applying the photodielectric resin layers containing ceramic powder on upper and lower surfaces of the substrate, laminating copper foils on the resin layers to form upper and lower electrodes, forming circuit patterns, and then etching the photodielectric resins, thereby obtaining a discrete capacitor.
Third is a method of manufacturing an embedded capacitor, including separately inserting a dielectric layer having capacitance properties into inner layers of a PCB, so as to substitute for a decoupling capacitor mounted on a PCB, which has been patented by Sanmina Co. Ltd., USA.
In this regard, U.S. Pat. No. 5,079,069, granted to Sanmina Co. Ltd., USA, discloses an embedded capacitor PCB including two conductive layers and an insulating material layer (dielectric layer) inserted into the two layers, and a PCB including the embedded capacitor PCB.
The dielectric layer of the embedded capacitor PCB has a thickness of at least 0.5 mil (12.5 μm), and generally, 1-2 mil (25-50 μm). The conductive layer has a weight distribution of at least 0.5 oz./ft2 (thickness: about 18 μm), and generally, 1 oz./ft2.
In addition, U.S. Pat. Nos. 5,155,655 and 5,161,086, granted to Sanmina Co. Ltd., USA, disclose the use of a conductive layer obtained by surface treating a conductive foil to assure adhesion and capacitance density.
In addition, U.S. Pat. No. 5,162,977, granted to Sanmina Co. Ltd., USA, discloses a capacitive core element using high capacitance powder. In the above patent, pre-fired ceramic powder having a high dielectric constant of 500 or more (at least 10 times that of the epoxy) is contained in a dielectric layer, to increase decoupling ability.
However, such conventional techniques suffer because the capacitance is very low, thus decreasing actual usability. To solve the problems, although attempts to use a component material having high capacitance have been made, they have not yet reached desired product requirements.